1. Field of Invention
The invention relates to an electric power steering system including a steering shaft which turns along with a steering wheel operated by a driver; a steering mechanism which connects the steering shaft to steered wheels of a vehicle, and which converts a turning motion of the steering shaft into a linear motion, thereby steering the steered wheels of the vehicle; an electric motor which applies force for assisting a turning operation of the steering wheel; a steering angle sensor which detects a turning angle of the steering shaft; and a drive control unit which controls driving of the electric motor based on the turning angle detected by the steering angle sensor.
2. Description of Related Art
An electric power steering system with a learning function has been well-known, for example, as disclosed in Japanese Patent No. 2970351 (JP-B-2970351). The electric power steering system determines whether a vehicle is running straight based on results of determination concerning a vehicle speed condition, a torque condition, and a steering angle condition. When it is determined that the vehicle is running straight, the electric power steering system averages output values which are obtained from the steering angle sensor the predetermined number of times that sampling is performed, thereby calculating an average value of the output values. The calculated average value is compared with the predetermined number of other stored average values. The average values selected by the comparison are further averaged, whereby a midpoint output value indicating a midpoint of the steering angle (hereinafter, referred to as a “steering angle midpoint”) is set.
However, in the above-mentioned type of electric power steering system, whether the vehicle is running straight is determined based on whether steering torque detected by a steering torque sensor satisfies the torque condition. Accordingly, there is a possibility that whether the vehicle is running straight cannot be determined accurately. In the case where steering angle midpoint is deviated, for example, if returning control of a steering wheel is performed by an electric motor, the turning position of the steering wheel connected integrally with a steering shaft is deviated from the neutral position, and the vehicle is turned. If a driver turns the steering wheel to the neutral position so as to correct the deviation, and maintains the turning position of the steering wheel at the neutral position, the steering torque is constantly detected. Therefore, as an amount of deviation of the steering angle midpoint, that is, an amount of deviation of the turning position of the steering wheel from the neutral position increases, the value of the steering torque which is obtained by detection increases. Accordingly, even when the vehicle is running straight due to the correction made by the driver, it may be determined that the vehicle is not running straight. If whether the vehicle is running straight cannot be determined accurately, the situation where the steering angle midpoint cannot be accurately corrected continues.